Calcium serves many functions in the body, but the main one is to form the structures that give our bones and teeth their strength and shape. As we get older, the mineral content of our skeleton declines. Our bones start to thin out bit by bit, so they become less dense, more brittle, and more likely to break. When this thinning advances to a certain point, it's called osteoporosis. Each year in the United States there are 1.5 million bone fractures associated with osteoporosis, and 250,000 of those breaks will involve a hip.
For years, high calcium intake has been portrayed as one of the best things you could do to prevent osteoporosis and related fractures. Research results supported this view, although many of the studies were fairly small and short.
But when researchers started to crunch the data from large, prospective studies that followed people for many years, the benefits weren't so clear-cut. The ambiguity led to randomized trials of calcium to test what effect it might have on fracture rates.
The tide started to turn in 2005 when results from two British studies showed that calcium didn't prevent fractures — even when taken in combination with vitamin D. The next year, results from a large American trial, the Women's Health Initiative, showed that postmenopausal women who took a calcium–vitamin D combination were no less likely to break their hip than women who took a placebo pill, although the density of their hip bones increased slightly. In 2007, a Swiss and American team, including some researchers from Harvard, reported the results of a meta-analysis of over a dozen studies of calcium. They found no connection between high calcium intake, from either food or pills, and lower hip fracture risk. In fact, when they limited their analysis to four randomized clinical trials with separate results for hip fractures, they found that extra calcium increased the risk.
The whys and wherefores
While a certain level of calcium intake is undoubtedly important to keeping bones strong, amounts above that level might not do much good. One reason extra calcium didn't show any benefit in the Women's Health Initiative may have been because the women in that study were, on average, already getting over 1,000 milligrams (mg) daily. And they weren't all that unusual. Because of dairy products and calcium pills, many Americans are amply supplied with calcium.
Another theory: calcium in large amounts may interfere with absorption of phosphorus, which is also crucial to maintaining bone strength. Phosphorus deficiency isn't a major problem in well-fed populations, but it's possible that extra calcium pushes some people into it, especially if their diets don't include much protein.
Another factor may be that added calcium isn't all that beneficial if our vitamin D intake is low. The body needs vitamin D to absorb calcium, and depending on how you define it, between 30% and 60% of us have less-than-optimal levels of vitamin D in our blood.
So now what?
According to current recommendations, Americans over 50 are supposed to get 1,200 mg of calcium daily. Dr. Walter C. Willett, chair of the Harvard School of Public Health's nutrition department, believes that many Americans are getting more calcium than they need. In his opinion, 600 mg is probably enough for most people to keep their fracture risk low, but because extra calcium might be protective against colon cancer, he sees a daily intake of 600 to 1,000 mg as a reasonable goal.
Current guidelines say Americans in the 50-plus age bracket are supposed to get 400 to 600 international units (IU) of vitamin D. But a growing number of experts — including Dr. Willett — say that's not enough, and that 800 IU, or even 1,000, would not only benefit our bones but possibly prevent some cancers and other problems.
April 2008 update